1. Field of the Invention
This invention relates to sub-surface release plugs used in cementing of the outer casing annulus of a well bore, and more particularly, to a sub-surface release plug having a sealed collet release mechanism, venting between upper and lower plugs and having an inner sleeve such that pressurization of the well string is not applied to the plugs themselves.
2. Description of the Prior Art
Sub-surface release plugs positioned in the upper portion of a well casing below a casing hanger and attached to the lower end of a drill string are commonly used in cementing operations for cementing a casing annulus adjacent a shoe joint. Typically, a bottom plug of the assembly is released and cement is pumped into the casing above the bottom plug, forcing the bottom plug downwardly until it comes to rest at the upper end of the shoe joint. The bottom plug seals against the inner surface of the casing so that mud below the bottom plug and cement above the bottom plug are not mixed. Once the bottom plug has reached its lowermost position, the bottom plug is opened to allow cement to pass therethrough. The cement then passes through a float collar and/or float shoe and an opening at the lower end of the shoe joint into the casing annulus. A valve in the float collar and/or float shoe prevents reverse movement of the cement through the casing.
When the proper amount of cement has been introduced into the casing and drill string, a releasing dart or drill pipe plug is dropped into the drill string. The releasing dart engages a latching mechanism above the top plug, thus closing off the central opening of the top plug and releasing it from the drill string. The fluid pumped into the drill string forces the top plug, and the dart or drill pipe plug latched thereto, down toward the bottom plug, forcing the cement through the shoe joint.
The top plug stops when it contacts the bottom plug. Once the cement has set, the top and bottom plugs are drilled out of the casing.
One such prior art sub-surface release plug system is disclosed in Halliburton Services Sales and Service Catalog 43, pages 2424-2426.
One problem with the previous system is that when the ball is dropped to release the bottom plug, high pressure is exerted on the inner surfaces of the plugs. This release pressure can burst the top and bottom plug inserts if special materials of construction are not used. Such materials, such as heat-treated aluminum alloys, provide a much stronger plug, but the increased strength results in a harder, tougher alloy which is more difficult to drill out later. The sub-surface release plug assembly of the present invention provides an inner sleeve means against which the pressure is applied such that no pressure is exerted on the inside of either the top or bottom plugs. Thus, the plugs can be made of lower strength, softer aluminum alloys or even partially made of plastic. This results in a plug assembly which is much easier to drill out after the cementing operation.
Another problem with the prior art apparatus is that a complex double collet mechanism is used which is relatively expensive to manufacture. Also, a portion of the collet mechanism is exposed to the cement and possibly may become clogged during the cementing operation which will make it impossible to release the top plug. The present invention uses a single collet mechanism which is sealingly separated from the fluids in the drill string until immediately prior to releasing of the top plug.
Still another problem with the previous system is that it is difficult or impossible to assemble in the field. The design of the present invention is constructed such that the two plugs are easily assembled by simple threaded engagement. This also allows the present invention to be easily adapted to situations where only one plug is desired.
A further problem with the sub-surface release plug assembly of the prior art is that no venting is provided between the plugs because it was necessary for the assembly to withstand pressure on the inside thereof. The present invention provides vent means between the plugs and above the top plug which prevents premature release of the bottom plug and inversion of the sealing lips on the top and bottom plugs.
As indicated, it is sometimes desired to have only one plug rather than the two-plug design. In such cases, the single plug acts in substantially the same manner as the top plug described herein. The present invention which is easily assembled or disassembled provides an apparatus which is quickly adaptable to either a one or two plug configuration.